MERSEYSIDE has 4,000 registered heroin addicts, a user-population density 158 per cent higher than the average in England and Wales and twice as high as the next largest, in north-west London.

The reason, according to police, and health and social services agencies in Merseyside, is that many of their drug abusers have been taken out of secrecy and criminality and put into clinics, not court.

Every registered addict represents a clean needle, a potentially stabilised habit. HIV infection is rare on Merseyside, a quarter of the national average and only 7 per cent of that in north-west London. In Scotland, the number of heroin addicts is virtually the same as the incidence of HIV, 24 times greater than in Merseyside.

'There has been a steady growth in registered addicts to the point where we have the highest per capita rate in the UK,' Superintendent Derek O'Connell, head of the region's drug squad, said. 'We've reached people who needed to be reached, to be treated and counselled.'

Merseyside changed its strategy in 1987, persuading the regional health authority to provide extra resources and becoming the first force to champion the clean-syringe campaign.

'The heroin problem has stabilised and treatment rates are up,' Supt O'Connell said. 'The crime rate also peaked in 1987 and, since 1991, it has dipped below the national average.'

However, Merseyside has an increasing cocaine habit, as well as abuse of amphetamines and ecstasy. Supt O'Connell believes that it is in schools, and from the efforts of teachers, that the greatest preventive impact can in future be made on the prime age group for drug abuse, 16- to 19- year-olds.

The police and Liverpool education authority have agreed a new set of guidelines for schools, including a four-year long element in the curriculum.

'You can't put all your eggs in one basket. We have transposed the problem from a criminal one to a criminal- medical-educational problem,' Supt O'Connell said.

Merseyside's drug squad is not demanding government action on the drug problem. The 40 officers believe they have sufficient powers and the optimum strategy, including cautions for first-time offenders. Between 900- 1,000 first-time arrests a year are released with a caution, and research by Liverpool University revealed that 76 per cent commit no further offence.

Decriminalisation is not in the squad's vocabulary, but those seeking treatment are protected, and those straying experimentally over the threshold of legality are treated sympathetically.

'We look at the individual, and what can be done with the individual,' Supt O'Connell said. 'We try to encourage the first-time offender to take responsibility.'

Rather than treat users harshly, Merseyside prefers to target the dealers, and few forces have matched its record of spectacular raids. Only the Metropolitan police make more arrests and seizures.

Sergeant Alan Connor, of the Merseyside squad, said: 'We've developed smashing relationships with customs and the regional crime squad, who target the really big operators while we concentrate on the middle tier.'

The squad, which operates from anonymous headquarters in Kirkby, is usually immersed in four or five major investigations.

'It means researching their activities, monitoring their movements, seeing if they're living in a mansion while on the dole,' Sgt Connor said.

'Only when we've researched them thoroughly, will we put them under surveillance - it costs pounds 3,000 a day.

'The laws and powers are good. The only problem we have is with unused material, every scrap of paper and piece of tape which is not required for the prosecution but which the defence has to see.'